The conventional electrophotographic image forming apparatus has an electrostatic latent image bearing member for bearing electrostatic latent images. The electrostatic latent images on the electrostatic latent image bearing member are visualized with particles of toner supplied from the developing device.
There have been proposed a variety of developing devices. Among others, one typical developing device has a developing roller disposed to oppose the image bearing member and a supply roller disposed in contact with the developing roller but away from the image bearing member in which toner is supplied from the supply roller to the developing and then collected from the developing roller to the supply roller so that a certain amount of fresh toner is provided to a developing region between the image bearing member and the opposed developing roller.
JP 2007-145904 (A) discloses a supply roller of which the outermost peripheral layer is made of polyurethane foam. According to the supply roller, stress which acts on the toner between the contact region between the developing and the supply rollers is decreased due to the flexibility of the polyurethane foam to extend the life duration of the toner.
In the meantime, the current trend to pursue a high-resolution and high-quality image production needs smaller size toner particles. The small size toner particles have a low fluidity so that they tend to clog within the opened cells in the surface of the peripheral polyurethane foam layer of the supply roller. Also, the small size toner particles tend to adhere firmly to the surface of the developing roller. This results in that the supply roller becomes unable to collect the toner particles from the developing roller in a short period of use to leave ghost memories made of uncollected toner particles, which may afterwards be transferred onto the recording medium such as paper to deteriorate the quality of the resultant images.
One possible approach to this problem is to increase the frictional force between the developing and the supply rollers for enhancing the collecting ability of the supply roller by, for example, increasing the peripheral speed of the supply roller relative to the developing roller or increasing the contact width between the developing and the supply rollers in the peripheral directions thereof.
Disadvantageously, the increased frictional force can result in another increase of the stress on the toner particles in the contact region, which reduces the life duration of the toner and deteriorates the quality of the toner to result in an unwanted toner adherence on the non-image region on the recording medium.